@cocofyn These are my settings and works fine on my BT FTTP connection using my XE75 Pro. (these are screen snips from my Archer AX72, settings are the same - my Decos are in the cupboard!) I always...

@Cyero It would seem you have done everything you can to rule out all the issues and have proven that the Deco seems to be the issue, hopefully TP link will investigate this for you.

@Cyero If you get 10/10 then your IPV6 is working correctly, if speed is still slow then its an issue with the Deco I would think.

@Cyero Edit: Your last screen snip looks ok to me? Go to https://test-ipv6.com/ and see what you get? I'm with BT full fibre, they use a PD of 56 however I see a PD of 64 on my LAN side, IPV6...

@Cyero no worries, and for "assigned type" have you tried "DHCPv6" instead of "SLAAC+RDNSS"?

@Cyero Just a thought, have you tried to change IPV6 internet settings "Get IPV6 address" from "auto" to "non address" to see if that makes a difference? Cheers